The present invention relates to a planar antenna that is small in size and low profile.
As a conventional planar antenna having a small size and low profile, an M-type antenna having a flat radiating electrode is disclosed in Japanese Patent Publication No. 5-136625A, which will be described with reference to FIGS. 35 to 37.
In the conventional M-type antenna as shown in FIG. 35, a radiating electrode 12, which is formed of a flat conductive plate and whose planar outer shape is square, is disposed to be spaced apart from a grounding plate 10 and parallel to the grounding plate 10. A feeding pin 14 is erected from the side of the grounding plate 10 and is electrically connected to an approximate center portion of the radiating electrode 12. In addition, at approximately symmetrical locations relative to the location where the feeding pin 14 is disposed, a pair of short pins 16 are provided such that center locations of outer edge portions of two opposing sides of the radiating electrode 12 are electrically connected to the grounding plate 10. The feeding pin 14 is electrically isolated from the grounding plate 10. In a case where a length of one side of the radiating electrode 12 is set to 84 mm and the height of one side of the radiating electrode 12 from the grounding plate 10 is set to 25 mm, a resonance frequency of about 900 MHz is obtained, as shown in FIG. 36. Further, in a case where the length of one side of the radiating electrode 12 is set to 84 mm and the height of one side of the radiating electrode 12 from the grounding plate 10 is set to 31 mm, a resonance frequency of 885 MHz is obtained, as shown in FIG. 37. The frequency of 885 MHz is a center frequency for the PDC 800 MHz band that is one of frequency bands used in cellular phones.
As described above, in the conventional M-type antenna, when the height by which the radiating electrode 12 is spaced apart from the grounding plate 10 is increased, a resonance frequency is decreased. As the result of simulation of current distribution of the M-type antenna, it could be understood that a current rarely flows at the sides where the short pins 16 of the radiating electrode 12 are not provided, while a large amount of current flows through the feeding pin 14 and the short pins 16 so as to resonate in a common mode. Accordingly, in a case where the height by which the radiating electrode 12 is spaced apart from the grounding plate 10 is increased, lengths of the feeding pin 14 and the short pins 16 are increased. As a result, a current path length is increased, and a resonance frequency is decreased.
However, in order to decrease the resonance frequency, the height by which the radiating electrode 12 is spaced apart from the grounding plate 10 should be increased. In a case where such an antenna is incorporated in a casing of an electronic apparatus where a small size and low profile is required, there is a drawback in that the height of the electronic apparatus is increased. Accordingly, it is required in achieving the small size and low profile of the antenna with low resonance frequency, without increasing the height by which the radiating electrode 12 is spaced apart from the grounding plate 10, and without expanding a planar shape of the radiating electrode 12.
Further, in recent years, an electronic apparatus has various functions that make users various media or services available. For this reason, a plurality of antennas may be needed, but an installation space of the antennas is generally restricted. When a separate antenna is additionally mounted in the conventional M-type antenna, the additional antenna is provided aside the radiating electrode 12 or on the radiating electrode 12. As a result, the large installation space is needed or the height is increased. Even when the plurality of antennas need to be provided, it is preferable that the arrangement space be as small as possible and the height be as low as possible.